Raman spectroscopy has been demonstrated to be a powerful non-invasive analytical technology for material characterization and identification. However, the strong fluorescence emission stimulated by the excitation laser often overwhelms the weak Raman signal, especially for composite materials. Several techniques have been proposed before to suppress the influence of the fluorescent emission. In one approach, the wavelength of the excitation laser is shifted to near-infrared (NIR) region as disclosed by Fujiwara M, et al. in Applied Spectroscopy, Vol. 40, p. 137, 1986. However, the Raman signal also becomes weaker, since the Raman scattering cross-section is inversely proportional to the fourth power of excitation wavelength. Another approach uses deep UV laser for Raman excitation as disclosed by Bowman W D, et al. in Journal of Raman Spectroscopy, Vol. 9, p. 369, 1980. But the lasers at this wavelength are both bulky and expensive.
Other approaches employ some laser modulation techniques. For example, by taking advantage of the fact that fluorescence emission and Raman emission have different decay times, the two spectra can be separated in the time domain by stimulating the material with an ultra short pulse laser as disclosed by Howard J, et al in Journal of Physics E: Scientific Instruments, Vol. 19, p. 934, 1986. This approach requires the pulse width of the laser to be in the order of pico-seconds. Commonly a nonlinear Kerr gate is used to separate the fluorescence emission from the Raman signal. Another approach, which is named as ‘shifted excitation Raman difference spectroscopy’ (SERDS), is proposed by Shreve A P, et al. in Applied Spectroscopy, Vol. 46, p. 707, 1992. In this approach, two similar Raman spectra with a small shift in wavelength are measured using a tunable laser. The difference between the two spectra is used to reconstruct the Raman spectrum. This approach utilizes the fact that the fluorescence spectra are generally independent of the excitation wavelength and its bandwidth, while Raman peaks occur at a fixed wavenumber distance from the excitation band and mimic its wavelength distribution exactly. A simpler but less effective approach is proposed by S. E. J. Bell, et al. in Analyst, Vol. 8, p. 1729, 1998. It obtains the difference Raman spectrum by shifting the position of the spectrometer, thus avoiding the use of the tunable laser.